The present invention generally relates to a scanning electron microscope for obtaining a scanning image by scanning an electron spot on a sample, and particularly to a scanning electron microscope capable of obtaining a scanning image with high spatial resolution within a low acceleration voltage region.
A scanning electron microscope has been conventionally used for observation and a length-measurement of submicron-order (1 micron or less), such as contact holes and line patterns in a semiconductor device sample.
The scanning electron microscope obtains a scanning image (SEM image) by scanning an electron beam emitted from a heating type or field emission type electron source on a sample to detect secondary particles such as secondary electrons and back scattered electrons as detection signals, and using the detection signals for the brightness modulation input for a cathode ray tube scanned synchronously with the electron beam scanning. In typical scanning electron microscopes, electrons emitted from an electron source are accelerated between the electron source to which a negative potential is being applied and an anode connected to the ground potential. The accelerated electrons are focused on the sample connected to the ground potential.
Very recently, scanning electron microscopes have been used in manufacturing process and inspections of semiconductor devices. For this purpose, a need has arisen for achieving high resolution below 10 nm with employment of such a low acceleration voltage less than 1,000 V in order to observe an insulating material without any charging-up effect.
That is, a semiconductor device sample is generally made by forming an electrical insulator such as SiO.sub.2 or SiN on a conductive portion of Al or Si. When an electron beam is applied to the semiconductor device sample, the surface of the electrical insulation is negatively electrified (hereafter may be expressed simply as "changing-up"), orbit of the emitted electrons is changed, and orbit of the primary electrons per se is changed. As a result, abnormal contrast or distortion may occur in the SEM (scanning electron microscope) image.
The above image interference due to charging-up seriously affects the observation of contact holes and the length-measurement of lines-and-spaces. Therefore, this makes it difficult to not only evaluate semiconductor manufacturing processes, but also assure the quality of semiconductor devices. For this reason, a so-called low voltage acceleration SEM has been conventionally used in which the energy of the primary electron beam which scans on a sample is 1 keV or lower.
However, the above-described prior art owns the following various problems. That is, if the acceleration voltage would become lower, observation at a high magnification would become difficult because the resolution is extremely degraded due to increase of chromatic aberration caused by energy spread of electron beams. If electron current would decrease, the ratio of secondary signal to noise (S/N) would extremely decrease, the contrast of an SEM image is impaired, and observation at high magnification and resolution would become difficult. Especially, for a semiconductor device made by an ultra-fine processing technique, signals generated from recessed portions of contact holes and line patterns become weak. Therefore, this makes fine observation and length-measurement very difficult.
To solve such a problem, some methods have been proposed. For instance, in accordance with proceeding of IEEE 9th Annual Symposium on Electron, Ion and Laser Technology, pages 176 to 186, the acceleration voltage applied between the electron source and the anode biased at the ground potential is set to the high value, and the retarding static field is produced between the objective lense biased at the ground potential and the inspection sample to which the negative potential is applied, so that the electron beams irradiated to the sample are decelerated. Thus, the acceleration voltage is eventually set to a relatively low voltage, and such a scanning electron microscope is proposed that both chromatic aberration can be reduced and the charging-up can be prevented.
Nevertheless, since the applications of negative potentials to the sample are manually performed by operators in this prior art scanning electron microscope, there are some possibilities that a shortcircuit happens to occurs between the sample and the ground potential when operators would forget to cut off the application of negative potentials to the sample during substitution of the sample. Moreover, there are possibilities that samples such as semiconductor devices which are readily, adversely influenced by electric damages may be destroyed due to sudden potential changes caused by the shortcircuit. As a consequence, there are various problems. That is, a careful operation is required when the voltage application is turned ON/OFF while the sample is mounted and replaced in the scanning electron microscope, i.e., a difficult handling of the sample is needed.